Judicial Watch has asked US District Judge Emmet G Sullivan for permission to question Mrs Clinton as part of a long-running lawsuit.

At issue is whether the State Department conducted an adequate search of public records in response to a Freedom of Information Act (FOIA) request.

The group has filed several lawsuits seeking records from government agencies related to Mrs Clinton's tenure as the nation's top diplomat from 2009 to 2013.

Mr Kendall said Mrs Clinton has already answered questions to the FBI as part of its now-closed criminal probe into whether classified material was mishandled, as well as to the congressional committee investigating the 2012 Benghazi attacks that killed four Americans.

Mr Sullivan previously approved requests for several of Mrs Clinton's closest aides to submit to questions, including for Chief of Staff Cheryl Mills and Deputy Chief of Staff Huma Abedin.

Judicial Watch lawyers pressed the current and former State Department officials on whether Mrs Clinton's exclusive use of private emails amounted to an effort to sidestep public records requests.

Mr Kendall dismissed those concerns in his argument, saying there was "no evidence that the purpose of the clintonemail.com system was to thwart FOIA".

He cited recent comments from FBI director James Comey to back up Mrs Clinton's own statements that she used the private server simply as a matter of convenience.

Mr Comey said last week that Mrs Clinton took that line in her interview with FBI agents.

Mr Kendall said that Mrs Clinton's comment in a 2010 email that she worried that any use of a State email account might risk the "personal being accessible" did not need to be explored further in a deposition because it had already been explained by Ms Abedin, who said Mrs Clinton did not want to expose any of her private emails.

"The meaning of Secretary Clinton's statement is obvious on its face," Mr Kendall said.

Mr Kendall also echoed Mrs Clinton's own previous contention that she thought her "emails were being captured and preserved in the department's record-keeping systems".

He added that even if she was mistaken, her use of the private account was "transparent to State Department officials, including those responsible for records management".

State department officials previously testified that they were vaguely aware that she was messaging with them from a private account, but emphasised that they were unaware she was exclusively using private emails or a private server to conduct all government business.

The department's records officials did examine accounts of Clinton aides and other agency officials to find other Clinton emails, but only after it became known that Mrs Clinton had turned over nearly 33,000 emails after disposing of a similar number she and her lawyers determined were personal.

Mr Comey said his agents had recovered "hundreds" of work-related emails that Mrs Clinton failed to provide, which she apparently deleted.

Mr Kendall cited a 1980 case involving State Department records held by former secretary of State Henry Kissinger, who had removed transcriptions of agency telephone recordings.

The Supreme Court ruled in the case that even if Mr Kissinger's removal of the records violated the Federal Records Act, the district court in the case lacked authority to order their return to the State Department.

Mr Kendall did not mention the politically-freighted possibility that a video deposition of Mrs Clinton could end up being used against the candidate in media ads by political action committees supporting Donald Trump.

But in a footnote, Mr Kendall said that if the judge does decide to allow a deposition, he urged Mr Sullivan "to allow Secretary Clinton to provide information in writing".

"Requiring her to sit for a deposition for the purpose of repeating her prior statement or stating that she has no knowledge of certain topics would serve no useful purpose," Mr Kendall wrote.

Mr Sullivan has scheduled a July 18 hearing to hear arguments on whether he should allow Judicial Watch to depose Mrs Clinton.

In a separate legal filing, the State Department said it also opposes allowing depositions by Mrs Clinton and two other officials.

Government lawyers told Mr Sullivan that Judicial Watch "has not revealed a shred of evidence indicating an intent" by the agency to evade federal public records laws.

The State Department also revealed in its brief that it has asked the FBI to turn over several thousand work-related emails that investigators were able to recover from Mrs Clinton's server.

On July 8, under-secretary for management Patrick Kennedy - who also was deposed in the case - asked Mr Comey to provide the previously-unreleased messaged to the State Department.

Mr Kennedy did not say in the letter whether the emails would be made available to the public.